simplifying to bivariate regression formula

I want to show that if the number of columns for my matrix of regressors is two and if the first column of my regressors matrix is full of ones then the OLS estimator of the second element of reduces to the bivariate regression formula.

How can I show such a thing? It seems obvious to me that is the same as something like although I'm not sure that's a solid enough "proof". Help!